1. Field of the Invention
This invention relates to crystalline boehmitic aluminas the crystallites of which exhibit unusual dimensional differences in the space directions 020 and 120. This invention further relates to a method for preparing such aluminas and the follow-up products obtained therefrom by calcination.
2. Description of the Prior Art
The structural relations of the various aluminium oxides and aluminium hydroxides are very complex. Main distinctions are made between α-Al2O3 (corundum), α-AlO(OH) (diaspore), α-Al(OH)3 (occasionally also termed βAl(OH)3, bayerite, or bauxite dihydrate), γ-Al2O3, γAl(OH) (boehmite), and γ-Al(OH)3 (occasionally also termed α-Al(OH)3, gibbsite, hydrargillite). In addition, there exist numerous modifications thereof, particularly modifications of different aluminium oxides obtained by thermal degradation of the aluminium hydroxides or aluminium oxide hydrates. For instance, it is generally believed that boehmitic alumina will undergo the following conversion under the influence of temperature:Boehmite→γ(gamma)-Al2O3→δ(delta)-Al2O3→θ(theta)-Al2O3→α(alpha)-Al2O3 
In literature references there are no standardized designations for the various aluminium oxides, aluminium oxide hydrates (occasionally also termed aluminium oxide hydroxides), and aluminium hydroxides, particularly with respect to the preceding Greek characters. The term ‘boehmitic aluminas’ as used herein comprises boehmitic and pseudo-boehmitic aluminas.
Boehmitic aluminas are known. High-purity boehmitic aluminas can be prepared for example by controlled hydrolysis of aluminium alkoxides. The resultant aluminium hydroxide hydrogels crystallize for example in the form of the rhombic aluminium oxide hydrate crystallite (γ-AlO(OH), boehmitic alumina).
DE 38 23 895-C1 discloses a process for producing boehmitic aluminas with pore radii which can be adjusted in a controlled way from 3 to 100 nm. According to said process, the boehmitic aluminas are subjected to hydrothermal aging at a steam pressure from 1 to 30 bar (corresponding to a temperature from 100 to 235° C.) for 0.5 to 20 hours with agitation at a peripheral velocity from 1 to 6 s−1. Such aluminas and the boehmitic aluminas produced by other processes have crys-tallite sizes (measured on the 020 reflex) which are always smaller by at least 2 nm compared to the crystallite sizes measured on the 120 reflex. In U.S. Pat. No. 3,898,322, too, a process for producing hydrothermally aged alumina suspension is described. According to said process, the aqueous aluminium hydroxide/aluminium oxide hydrate suspension obtained by hydrolysis of the aluminium alkoxides is subjected to hydrothermal aging at room temperature for 2 to 60 hours.